https://leetcode.com/problems/swap-nodes-in-pairs/description/

Given a linked list, swap every two adjacent nodes and return its head.
For example,
Given 1->2->3->4, you should return the list as 2->1->4->3.

 1 public ListNode swapPairs(ListNode head) {
 2         //如果是最後一對的話，head.next.next 是可以為 NULL 的，所以不要進行判斷
 

 3         if (head == null || head.next == null) return head;
 4         //base
 5         ListNode newHead = swapPairs(head.next.next) ;//head is 1, newHead is 4
 6         //currently head is 1,   4->3->null
 7         ListNode tail = head.next ; //tail is 2
 8         head.next = newHead ; //1->4
 

 9         tail.next = head ; //2->1
10         return tail;
11     }

2-1-4-3-5

5 自己就彈了，所以沒有後面的處理，所以(tail=3)tail.next 還是 直接連 last pop head(5) 所以是 2-1-4-3-5

LC_24. Swap Nodes in Pairs